Add zPages usage instructions, remove unneeded details

[jajanet]

• Add usage instructions for Bazel, including how to run the example
• Add TODO for making CMake instructions
• Remove most of the zPages explanation, replacing it with a link to the zPages spec

[codecov]

Codecov Report

Merging #288 into master will increase coverage by 0.49%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #288      +/-   ##
==========================================
+ Coverage   93.93%   94.43%   +0.49%     
==========================================
  Files         146      146              
  Lines        6564     6596      +32     
==========================================
+ Hits         6166     6229      +63     
+ Misses        398      367      -31     

Impacted Files	Coverage Δ
.../opentelemetry/sdk/metrics/aggregator/aggregator.h	58.82% <100.00%> (+2.57%)	⬆️
...emetry/sdk/metrics/aggregator/counter_aggregator.h	96.15% <100.00%> (+0.32%)	⬆️
...elemetry/sdk/metrics/aggregator/exact_aggregator.h	97.56% <100.00%> (+0.12%)	⬆️
...elemetry/sdk/metrics/aggregator/gauge_aggregator.h	100.00% <100.00%> (ø)
...etry/sdk/metrics/aggregator/histogram_aggregator.h	96.55% <100.00%> (+0.12%)	⬆️
.../metrics/aggregator/min_max_sum_count_aggregator.h	100.00% <100.00%> (ø)
...lemetry/sdk/metrics/aggregator/sketch_aggregator.h	69.76% <100.00%> (+0.71%)	⬆️
...clude/opentelemetry/sdk/metrics/sync_instruments.h	94.77% <100.00%> (+23.78%)	⬆️
sdk/test/metrics/metric_instrument_test.cc	100.00% <100.00%> (ø)
... and 1 more

[reyang]

LGTM.